1. Field of the Invention PA1 2. Description of the Prior Art
This invention relates to a metal gasket used for sealing fitting surfaces between a cylinder head and a cylinder block in a multiple-cylinder engine.
A metal gasket made from a metal material has been used conventionally for sealing a cylinder head and a cylinder block in an engine. The metal gasket has beads near through-holes correspondings to cylinder bores, water passages and oil passages, and when the cylinder head and the cylinder block are fastened and fixed by bolts, beads form elastic annular contact portions to the opposed fitting surfaces and seal them.
In engines of the recent types, however, a higher output and a smaller weight have been required, and an aluminum material having a smaller specific gravity tends to be employed in place of conventional materials having a great specific gravity such as steel and castings so as to produce the cylinder head, the cylinder block, and so forth. The aluminum material has a small weight but has low rigidity. Therefore, relative displacement of the cylinder head to the cylinder block tends to become great during the operation of the engine. When the opposed fitting surfaces of these structural materials are fastened by bolts through the single plate metal gasket, the opposed fitting surfaces are likely to be non-uniform because the bolt positions are dispersed relatively to the outer peripheral portions of the metal gasket and because they are not always distributed uniformly to the periphery of cylinder bores, that is, holes for combustion chambers. As a result, a combustion gas having a high temperature and a high pressure enters the gap between the opposed fitting surfaces at positions having a large distortion, such as in the regions between the holes for combustion chambers, and corrodes and contaminates the beads of the metal gasket, lowering thereby the seal effect.
In the case of the metal gasket, the gap between the cylinder head and the cylinder block repeatedly increases or decreases during the combustion cycles of the engine, and stresses or in other words, mechanical stress and thermal stress, repeatedly act on the metal gasket. This load change stress occurs as a great value at the portions having the lowest rigidity of the cylinder block and the cylinder head, so that the beads undergo buckling, cracks occur, and seal performance is deteriorated.
A metal gasket disclosed in Japanese Patent Laid-Open No. 293363/1988 is shown in FIG. 11. As shown in the drawing, two intermediate plates 65, 66 are laminated between elastic metal plates 61, 62 having beads 63, 64 so formed as to extend along the periphery of each hole 71, and spacer members 67, 68 are clamped between the two intermediate plates 65, 66 at portions closer to the hole 71 than the contact portion of the beads 63, 64. Compensation portions 69, 70 are disposed so as to compensate for any non-uniformity between the deck surfaces. In the metal gasket, the spacer members 67, 68 are clamped between the two intermediate plates 65, 66 at the portions closer to the hole 71 than the contact portion of the beads 63, 64. Therefore, one additional contact surface is provided to bear the seal force. The spacer members 67, 68 must be fixed to the intermediate plates 65, 66 by means such as welding, bonding, and so forth.
The metal gasket disclosed in Japanese Patent laid-Open No. 65367/1989 laminates two intermediate plates between two elastic metal plates forming beads along the periphery of holes. The edge of one of the intermediate plates is held in a grommet form by the edge of the other intermediate plate, and compensation bent portions are disposed on both surfaces so as to form substantially the same offset portions on both surfaces. In this metal gasket, the edge of one of the intermediate plates is held in the grommet form by the edge of the other, substantially the same offset portions are formed on both surfaces, and the bent portions for compensation are disposed. When they are clamped by bead substrates on both sides and are compressed under the buckled state between the cylinder head and the cylinder block, the turnup offset portions held in the grommet form function as the seam of the beads. As a result, the stress amplitude of the elastic metal plate drops, but the bending stress and the stress amplitude occur at the compression stop portions of the intermediate plates due to the difference of the operations of the upper and lower elastic metal plates.